The diet of humans and livestock is based largely on plant material. However, crop plants are generally rendered as low nutritional quality food sources because they contain low proportions of several amino acids which are essential for, but cannot be synthesized by, monogastric animals. Therefore synthetic essential amino acids are usually supplied as supplements to grain-based and other diets, in order to increase their nutritional value.
Two of the essential amino acids needed for animal nutrition often missing from crop plants are threonine and lysine. Various attempts have been made in the past to increase the levels of free threonine and lysine in plant tissues by breeding and by mutant selection. In addition, there were attempts to change the composition of the storage proteins accumulated in crop plants, but with minimal success. Either the expression of transgenic storage protein was too low or there were unacceptable quality changes in the product.